Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
This paper reports on a research program at Georgia Tech to utilize spherical surface near-field measurements in the evaluation of the electromagnetic performance of radomes. Near-field measurements are performed on a spherical surface which encloses a non-spherical radome. A backward transform technique has been developed in which the spherical near-field measurements are used to determine the field on the outer surface of the non-spherical radome shape. This backward transformed field on the outer surface of the radome is compared to the backward transformed field at the same locations without the radome in place. Measured data is presented for a tangent ogive radome. The data shows the point by point phase characteristics of the radome wall. Several additional measurements are displayed showing the ability to detect small dielectric patches attached to the outer surface of the radome, simulataing radome defects.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
A near field focus procedure for image processing using near zone backscattered fields obtained in a compact range is presented in this paper. An array of defocussed feeds is used to illuminate a target in a compact range. The backscattered field received at each feed antenna with the target being stationary is compensated by a phase factor. These compensated signals are then summed coherently to obtain a cross range image of the target which indicates the location of the various scattering centers associated with the target. Numerical examples are presented to validate this technique.
The Systems and Techniques Laboratory (STL) of the Georgia Tech Research Institute is upgrading their measurement capabilities by developing a planar nearfield range. The new range incorporates a Microvax Workstation II, an Hewlett Packard 8510B, and an automated planar scanner designed and developed by STL. The scanner is automated using an IBM compatible personal computer while Microvax serves as system controller on the IEEE 488 bus which interconnects the system.
The Systems and Techniques Laboratory of the Georgia Tech Research Institute is producing a PC-controlled near-field planar scan system which will allow phase measurements more accurate than one degree at 10 GHz in a 10 foot by 12 foot plane. This high degree of accuracy will be accomplished with microstep motors, absolute linear encoders, and a helium neon laser compensator. The probe positioning system consists of a tower traveling across a set of linear rails. A probe moves vertically on the tower, allowing operator pre-described measurements to be taken. The system is designed to accept data as the probe moves vertically, then indexed horizontally for complete plane coverage.
For the past two years the National Bureau of Standards (NBS) has been developing the capability to perform on-axis gain and polarization measurements at millimeter wave frequencies from 33-65 GHz. This paper discusses the error analysis of antenna measurements at these frequencies. The largest source of error is insertion loss measurements. In order to make accurate insertion loss measurements, flanges on antennas need to be flat and perpendicular to the waveguide axis to within approximately 0.001 cm (0.0005 in). In addition, waveguide screws need to be tightened with a device that supplies constant torque. For antennas with gains less than about 25-30 dB (probes) we can measure on-axis gains within an uncertainty of 0.14 dB in the 33-50 GHz frequency band and within 0.16 dB in the 55-65 GHz frequency band using the three-antenna technique on the extrapolation range. For antennas with larger gains we can measure on-axis gains within an uncertainty of 0.21 dB in the 33-50 GHz frequency band and within 0.24 dB in the 55-65 GHz band using the planar near-field technique. NBS in continuing development of its measurement capabilities, including measuring probe correction coefficients required in planar near-field processing, in order to provide accurate pattern measurements at these frequencies.
While near field antenna test techniques are well understood, published methods for high volume testing are rare. This paper addresses special requirements for production testing of satellites at the Hughes Aircraft Company Space and Communications Group facility in El Segundo, California. The El Segundo facility has the capability of testing antennas which employ multiple beams and polarization isolation for frequency spectrum reuse. It is required that the measurement techniques and equipment be able to test this type of antenna during a single traverse of the planar near field scanner. Serious demands are placed on the system to meet these requirements: * Maximum dynamic range and linearity must be maintained in an environment of rapidly shifting signal levels. * Isolation of signals must be maintained while allowing rapid switching for beam and polarization sampling. * Equipment settling time must be minimized to maintain scan rate at the highest possible speed. * RF interfaces must be repeatable, and capable of rapid reconfiguration. * Calibration and system checkout techniques must be accurate, quick, and capable of detecting malfunctions and costly setup errors. * Data transfer and processing must not be a limitation to the availability of the system for measurement. * System growth capability must be maintained, but not allowed to interfere with 'old and valued' customers. Some of the trades and pitfalls in meeting these requirements will also be presented.
For over a decade the National Bureau of Standards (NBS) has used the planar near-field method to accurately determine the gain, polarization and patterns of antennas either transmitting or receiving cw signals. Some of these calibrated antennas have also been measured at other facilities to determine and/or verify the accuracies obtainable with their ranges. The facilities involved have included near-field ranges, far-field ranges, and compact ranges. Recently, NBS has calibrated an antenna to be used to evaluate both a near-field range and a compact range. These ranges are to be used to measure an electronically-steerable antenna which transmits only pulsed-cw signals. The antenna calibrated by NBS was chosen to be similar in physical size and frequency of operation to the array and was also calibrated with the antenna transmitting pulsed-cw. This calibration included determining the effects of using different power levels at the mixer, the accuracy of the receiver in making the amplitude and phase measurements, and the effective dynamic range of the receiver. Comparisons were made with calibration results obtained for the antenna transmitting cw and for the antenna receiving cw. The parameters compared include gain, sidelobe and cross polarization levels. The measurements are described and some results are presented.
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.
A planar near-field scanner is described. It has an effective scan plane of more than 5 by 12 meter. The scanner will be used for the measurement of the Synthetic Aperture Radar (SAR) antenna of the European Remote Sensing satellite ERS-1. The requirements are discussed and the results of the first mechanical verification measurements are presented.
Near-field antenna measurements have many advantages, but also some limitations, which can be mainly attributed to the need for costly facilities or severe environmental effects. Although anechoic chambers are widely employed, absorbing material is very expensive and the whole construction becomes a considerable project, especially if it is required to accommodate various size antennas over wide frequency ranges.
Near-field testing of a very low sidelobe, L-band, 32-element, linear phased array antenna was conducted. The purpose was to evaluate testing and calibration techniques which may be applicable to a much larger, space borne phased array antenna. Very low sidelobe performance in a relatively small array was achieved by use of high precision transmit/receive modules. These modules employ 12-bit voltage controlled attenuators and phase shifters operating at an intermediate frequency (IF) rather than at RF. Three array calibration techniques are discussed. One technique calibrates the array by means of a movable near-field probe. Another method is based on mutual coupling measurements. The last technique uses a fixed near-field source. The first two calibration methods yield substantially the same results. Module insertion attenuation and phase can be set to 0.02 dB and 0.2 degrees, respectively. Near-field measurement derived antenna patterns were used to demonstrate better than -20 dBi sidelobe performance for the phased array. Application of increasing Taylor array tapers showed the limitations of the measurement systems to be below the -35 dBi sidelobe level. The effects of array ground plane distortion and other array degradations are illustrated.
A pulsed, coherent radar system was used in the inverse synthetic aperture radar mode to obtain 1-way high resolution images of simple antennas. These high resolution images display the amplitude and phase distribution of the received wave. The images were then edited and reconstructed using System Planning Corporation's Image algorithms contained in the SPC RPS software package. The 2-D (range vs. cross range) image data is very useful for detecting defects in antennas and can also 0be applied to modification of illumination conditions such as wavefront sphericity (phase taper) and/or amplitude variabilities (taper, ripple). This technique offers an alternate approach to near field/far field transformation. The technique involves rotation of the antenna under test at a controlled, uniform rate. The antenna port is connected to the radar receiver and the radar transmitter attached to an illuminating antenna. The radar transmits a step chirp wave form. The received signal is recorded to tape and processed off-line on the SPC Image Reduction Facility. A calibration technique was developed using simple wide bandwidth horn antennas. The downrange and cross range resolution of these 1-way ISAR antenna images is half as large as with 2-way radar ISAR for the same bandwidth and angular integration interval. Image data will be shown on reflector-type antennas to illustrate the technique.
There is renewed interest in the idea of determining the near-zone and far-zone bistatic RCS of complex targets from near-field data. This paper addresses the issue of efficient acquisition and processing of the requisite scattered near-field electric field data for determining the wide-angle bistatic RCS of electrically-large targets. Toward that end, several potential combinations of target illumination and near-field scanning techniques are considered in this paper. The techniques considered encompass mechanical and electronic scanning methods using single probes, linear probe arrays, and planar probe arrays to accomplish the near-field scanning, combined with either (a) compact range illumination or (b) "synthesized" plane wave illumination employing a single probe, a one-dimensional (1-D) probe array, or a two-dimensional (2-D) probe array. A general Spherical Angular Function (SAF) integral formulation of near-field bistatic coupling/scattering is presented, and an approximate "deconvolution" technique for electrically-large targets is described.
A spherical backward transform technique has been developed and applied to the determination of radome anomalies from near-field measurements. This paper reports on this technique and presents measured data for a missile radome.
The techniques of near-field antenna pattern measurement can be extended to near-field RCS measurement. The motivation for doing so is precisely the same as that for near-field antenna measurements; i.e., the convenience of an indoor antenna range, and an improvement in accuracy. Although the near-field measurement problem is solvable in principle in a manner analogous to the near-field antenna problem, it requires a significantly larger amount of time to take the necessary data, and to subsequently process the data to obtain useful quantities. BDM is currently involved in an on-going program to evaluate the feasibility of near-field bistatic RCS measurements. At the time of this writing, a complete set of mathematics has been formulated to handle the probe correction and data processing. The hardware has been built, software development is near completion, and the analysis of canonical scattering objects has been completed. Experimental data soon to be taken for these objects will be presented. It is hoped that the technique will prove to be a practical approach to RCS measurements.
For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.
A 15-meter diameter self-deployable antenna has been developed which utilizes the hoop-column structural concept with a gold-plated molybdenum mesh reflector. This antenna was developed to determine if a system could be designed and built with the dimensional tolerances necessary for in-space operational performance and for use as a test article in a ground based technology development program. One feature of the design is the provision for reflector surface shape control by cable adjustment. The antenna was deployed and tested at the Martin Marietta Denver Aerospace Near-Field Test Laboratory to measure its surface shape and its electromagnetic performance. RF test results show very good agreement between predicted and measured radiation patterns. The antenna is currently undergoing modifications which will allow automated surface adjustments and adaptive feeds to be utilized for further improvement in the electromagnetic performance. Controls, structural, and simulated thermal deformation tests will be integrated with future electromagnetic tests.
In any type of electromagnetic measurements, the ideas of "precision and accuracy" and "low cost" tend to be mutually exclusive. At Scientific-Atlanta, for instance, production testing of antenna products is conducted in low cost miniature "anechoic chambers" which are fabricated in-house. These "chambers" are actually medium-sized to large (64-200 cubic feet) rectangular boxes with absorber attached to their walls. They are usually equipped with single axis positioners at one or both ends, and their usefulness is limited to the measurement of axial ratio on low gain small antennas.